Calgary Gardening Adventures Roofing and Soil Quality

Calgary Gardening Adventures Roofing and Soil Quality

Thursday, October 29, 2009

Roofing and Soil Quality

Lately we have been pondering the potential impacts, real or imagined, of various roofing materials and potential impacts to garden soil quality.

Because about 80% of our watering supply is from roof-fed rainbarrels into a very intensive small-sized vegetable plot, the roofing material we choose to replace our battered asphalt shingles could theoretically have some long-term impact on our garden soil quality. However, there does not seem to be much readily available data on the topic (or at least not that I’ve yet found).

I doubt the grit and gravel off the asphalt shingles will do your garden any harm. Find out what glue is used to hold it onto the shingle. Might give you some clue as to potential toxicity problems.

Not sure what metal roofing sounds like in a rain storm. It is increasingly popular here as a replacement for the old grey asbestos slabs that have been used since Soviet times.

It is not the grit and gravel that is in question, but the hydrocarbons that leach out of an asphalt roof over time. Certain hydrocarbons have high levels of toxicity and are very persistent in the environment.

Some of my coworkers have spent a lot of time at work dealing with removal of hydrocarbons from soil at contaminated sites (not surprising, there is a lot of hydrocarbons in Alberta soil). Many of the sites are not even agricultural land. It feels like wasted time when the contamination could have easily been prevented.

Makes me question why we would willingly put a hydrocarbon saturated asphalt roof on our home that will leach hydrocarbons (known toxins) into our vegetable garden, and then eat veggies which inevitably will have some soil attached to them, albeit in very small amounts. Are the amounts of toxins too small to have much affect? Possibly. But in light of uncertainty, if possible health hazards can be avoided (within reason), it seems prudent to avoid.

Just one factor of many as we weigh pros and cons for the various choices available.

Although in some places, orangy clay tile is used, in most of Australia, galvanized steel or steel coated with other materials (eg zinc-aluminium resins)is the typical roofing material. Steel roofs are very loud in a rainstorm and very creaky as they heat up in the sun. But you get used to it. I miss the thudding of raindrops in this bitumen shingle land.

Australia is also a place where rainwater is extensively used both for human consumption and for gardens. Steele roofs, rainwater collection, and steel tanks go together well.

I cant remember ever seeing the typical North American bitumen-gravel roofs in Australia. Cant say I especially like them and the gravel in the gutters and rainbarrels is annoying. I dont think that bitumen is water soluable, though, so Im not sure why there would be any worry about using water that runs off the roof. Id be more worried about any additional binding agents that might be added.

Dave: Asphalt shingles are likely popular in Canada mostly due to low cost since we are so close to petroleum extraction and refineries. Asphalt shingles contain about 20-40% asphalt by weight according to:

A straight-seam metal roof (with hidden screws) for our house is almost double the cost of premium 50-year asphalt shingles (materials & installation). Fancier metal roofs are even more.

The asphalt in the shingles most certainly leaches out over time, as described below by a major asphalt shingle manufacturer we are considering:

Calgary Gardening Adventures Roofing and Soil Quality

The life expectancy of asphalt shingles is based on the performance of three components, and their ability to resist weathering. Made from petroleum, asphalt contains oils that make asphalt shingles easy to work with and effective at protecting your roof and home. As time goes on, these oils come to the surface and are weathered away by the elements. Its this weathering process that ages a shingle.

The question is not does the asphalt leach out but rather is there any potential long-term negative health effects to asphalt leaching into the garden soil via rainwater collection. Probably no cause for concern, but would be interesting to know if someone conducted some studies on the topic. Its always nice to make a truly informed decision, though rarely possible.

Hi Middle Earth Garden:

I did an electronic literature search for research on any environmental effects of asphalt shingles, but came up with near zero. But Water Research Volume 35, Issue 17, December 2001pp. 4200-4207 has a review by Brandt & de Groot (Aqueous leaching of polycyclic aromatic hydrocarbons from bitumen and asphalt) that mentions roofs. To quote them, Few papers about aqueous leaching of PAHs from bitumen into water exist. In Brandt & de Groots tests they found leach water from bitumens [9 commercial bitumens and one asphalt made from bitumen] stay well below the surface water limits that exist in several EEC-countries and are also more than an order of magnitude lower than the current EEC limits for potable water.

So maybe no concern, although not much reseach. There are some recent papers on PAHs (polycyclic aromatic hydrocarbons) in fumes from hot asphalt and their danger to road workers and asphalt plant workers in Eastern Europe. So, maybe a bigger worry is breathing in your garden on a hot day.

Dave: Thanks for the info. In my trolling of the internet, I came to a similar conclusion. It is likely something that should be studied more in the future, but for now based on the limited data available it seems the health risk is very low.

I dont really think that the water stays on the roofing long enough to collect any residue hydrocarbons from your asphalt shingles. Rain usually means cooler temperatures and that would also mean less hydrocarbon leaching. If you are really concerned, test the water in your rainbarrels.

Testing the water is a good idea Ann. The hydrocarbons definitely leach out over time, this is what causes the shingles to shrivel and curl. I have considered testing the water and soil, more for curiosity sake than concern. However, Im fairly certain the amounts of hydrocarbons in the water or soil would be below the detection limits. The soil would be a better test for long-term cumulative effects.

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